This invention relates generally to optical communication systems for interconnecting a plurality of remotely located stations.
There are generally two types of communication systems using optical fibers. One is an active type system in which an optical signal received at a repeater at each station is changed into an electrical signal, and then is changed back into an optical signal. Another is a passive type system in which an optical signal received at each station is transmitted as it is, after being added with other optical signals, or being branched.
It is impossible to transmit information in the former system even if one of the repeaters fails or goes off. However, it is possible to communicate between two stations in the latter system if only the two stations are operating. Therefore, the latter system is employed in many cases.
There are roughly two types of known passive systems as described in U.S. Pat. No. 3,883,217. One system employs tee couplers 10 as shown in FIG. 1.
Another system employs a star coupler 12 as shown in FIG. 2. In the two figures, stations 14 each includes an electrical unit 16 for generating and processing information, and an optical converter 18.
Optical converters 18 change electrical signals from electrical units 16 into optical signals and vice versa. Optical transmission lines 20 transmit optical signals between tee couplers 10 and optical converters 18. Further, there are common optical signal transmission lines 22 in the system using tee couplers.
Since tee couplers 10 are inserted into a common optical signal transmission channel, the light loss increases proportional to the number of stations. Therefore, a system using tee couplers 10 is disadvantageous due to the limited number of stations 14 that can be interconnected as described in U.S. Pat. No. 3,883,217.
Since an optical signal passes only once through star coupler 12 in the FIG. 2 system, light loss does not increase with an increase in the number of stations 14 except for power splitting loss. However, optical signal transmission lines 20 become very long because each station 14 is connected to the common star coupler 12. Furthermore, an increase in the number of stations 14 raises the cost to lay optical signal transmission lines 20.
Yet another optical communication system is known as shown in FIG. 3. The system has plural sub star couplers 24 in addition to one main star coupler 26.
However, each star coupler 24 and 26 outputs an input signal to all ports equally. Thus, oscillation may occur between main star coupler 26 and any sub star coupler 24. This oscillation may be prevented by inserting between main star coupler 26 and each sub star coupler 24 a gateway which controls the passage of signals. However, the use of gateways makes the system complicated.
Also, each optical transmission way is branched as a tree. The number of stations is limited for this reason.